Producing nuclear fuel includes converting and enriching uranium ore concentrate, and then making it into a ceramic form, having the size of a little fingernail, which is called a sintered uranium dioxide pellet (UO2 pellet). Subsequently, several hundreds of the UO2 pellets are put in a metal tube, and the metal tube is sealed, thus forming a fuel rod. Several hundreds of the fuel rods are thereafter fastened to a strong metal structure, forming a bundle of nuclear fuel rods. Typically, about one hundred fifty bundles of nuclear fuel rods are placed in a nuclear reactor of a nuclear power plant and take part in a fission chain reaction, thus generating heat.
Making nuclear fuel for light-water reactors includes converting low enriched uranium hexafluoride (UF6) into uranium dioxide (UO2) powder. Methods of this conversion are classified into a wet process in which water makes contact with uranium, and a dry process in which vapor makes contact with uranium. Preferably, a dry conversion process may be used, because there is a financial advantage in that equipment is simple, so the manufacturing costs are low.
UO2 powder that has been produced by the dry conversion process is homogenized, and formed into a cylindrical powder compact, the length of which is 10 mm, after a powder preparation process. The powder compact is thereafter sintered at high temperatures ranging 1700° C. to 1750° C. and then ground to form a sintered pellet having a predetermined diameter. The sintered pellet is finally completed through cleaning and drying processes. The completed sintered UO2 pellet has a weight of about 5.2 g and a diameter of about 8.05 mm. Approximately 356 sintered UO2 pellets may be charged into each fuel rod, and 96,000 may be charged into each bundle of fuel rods.
FIG. 1 is a perspective view showing the external appearance of a cylindrical sintered UO2 pellet. A sintered UO2 pellet p is manufactured into a ceramic form that is hard and safe. The sintered UO2 pellet p has a cylindrical shape, which has a side surface s, an upper surface and a lower surface f and has a predetermined diameter.
If the sintered UO2 pellet p, especially, the side surface s thereof, is defective, it may not be used as nuclear fuel. Therefore, inspection whether the surface of the UO2 pellet p is defective or not is required. Hereinafter, the term ‘surface’ refers to the side surface s′.
As described above, a large amount of sintered UO2 pellets are necessary in a nuclear plant. Thus, an apparatus that can rapidly and precisely inspect the surfaces of a lot of UO2 pellets is required.